Speech captured by a microphone may contain a noise component. If the captured speech contains a noise component, intelligibility of the speech may be reduced. In view of this, techniques have been developed for suppressing noise by estimating the noise component contained in the speech signal for each frequency band and by subtracting the estimated noise component from the amplitude spectrum of the speech signal (for example, refer to Japanese Laid-open Patent Publication Nos. H04-227338 and 2010-54954).
However, if, for example, a vehicle driver's speech is to be captured by a microphone mounted in a vehicle while the driver is driving with vehicle windows left open, the noise component contained in the speech signal may becomes larger than the signal component corresponding to the speech intended to be captured. In such cases, any of the above prior art techniques may suppress not only the noise component but also the signal component, resulting in reduced intelligibility of the intended speech.